docs: def getMessageSignature(self, message):
          """Return the signature of a message.
  
          If I am not a sign-all association, the message must have a
          signed list.
  
          @return: the signature, base64 encoded
  
          @rtype: str
  
          @raises ValueError: If there is no signed list and I am not a sign-all
              type of association.
          """
          pairs = self._makePairs(message)
          return oidutil.toBase64(self.sign(pairs))